Thermo-optic phase shifters control the properties of optical circuits by selectively heating sections of the optical circuit to control the optical phase of the light in the circuit. Thermo-optic phase shifters function by passing current through an integrated local resistor.
Thermo-optic phase shifters should have a relatively small footprint. The small foot print is needed because chips can experience thermal and stress gradients that can dynamically change the optical properties within waveguide circuits. The bigger the optical circuit footprint, the more susceptible it can be to dynamic temperature and strain profiles within the chip. This is particularly true in (but not limited to) circumstances where a single light source is split into two paths that are subsequently recombined to control the light through the optical circuit via optical interference.
An important aspect of the design of a thermo-optic phase shifters is to address the electromigration issue because thermo-optic phase shifters usually work with a significant electric current at elevated temperatures. Electromigration in a conductor is the result of momentum transfer from the electrons to the metal ions. Electromigration is a reliability concern with integrated circuits, especially those at scaled dimensions and high current densities, since excessive electromigration can lead to failures.
The efficiency of a thermo-optic phase shifter can be characterized by two figures-of-merit (FOM). The first FOM (FOM1) is the induced optical phase change by a certain heater power change. The second FOM (FOM2) is the induced optical phase change by a certain temperature change. Generally, the better the heater efficiency, the less heater power is needed and the less temperature excursion is required to meet a given thermo-optical phase shifter specification. Minimizing power consumption (maximizing FOM1) is important for scaling short reach link solutions, and large-scale thermal management within data centers and advanced computing systems. Minimizing the temperature excursions (maximizing FOM2) will improve the device's reliability in electromigration sensitive applications, because electromigration concerns scale exponentially with temperature.
Thermo-optic phase shifter configurations have been proposed to address these design considerations. For instance, U.S. Pat. No. 7,676,121 issued to Gill et al., entitled “Thermo-Optic Tuning of a Multi-Directional Optical Waveguide,” proposes heating elements that are looped into different segments to heat the optical waveguides, with a goal towards increasing efficiency, minimizing power consumption, etc. However, the design proposed in U.S. Pat. No. 7,676,121 has some notable drawbacks. The heating element is a resistor wire placed far away from the waveguide which transfers heat to the waveguide through low thermal conductivity material—thereby compromising efficiency. Further, the optical waveguides in the heating elements all have the same cross sections, and the thermal loading and heat transfer efficiency is not well controlled.
Therefore, improved thermo-optic phase shifter designs would be desirable.